This invention relates to an idling rpm feedback control method for internal combustion engines, and more particularly to a method of this kind which is adapted to control the engine rpm at engine idling even if maximum engine load is applied on the engine when an abnormality occurs in the throttle valve opening detecting system, so as to prevent engine stall.
In an internal combustion engine, the engine can easily stall due to a drop in the engine speed when the engine is operated in an idling condition at a low temperature of the engine cooling water or when the engine is heavily loaded with loads by head lamps, air conditioner, etc. in a vehicle equipped with the engine. To eliminate such disadvantage, an idling rpm feedback control method has been proposed e.g. by Japanese Patent Provisional Publication (Kokai) No. 55-98628, which comprises setting desired idling rpm in dependence upon load on the engine, detecting the difference between actual engine rpm and the desired idling rpm, and supplying supplementary air to the engine in a quantity corresponding to the detected difference so as to minimize the same difference, to thereby control the engine rpm to the desired idling rpm.
According to this method, when the throttle valve is not fully closed, that is, when the throttle pedal is being operated, it is not necessary to apply feedback control of the engine rpm, as the engine rpm can be adjusted in accordance with the throttle pedal operating manner of the driver. Thus, one of the conditions for applying feedback control during engine idling is that the throttle valve should remain fully closed. However, if any abnormality occurs in the throttle valve opening detecting means, the idling rpm control device can make a wrong diagnosis that the throttle valve is not fully closed, despite the engine being in an idling state with the throttle valve fully closed, and consequently the supply of supplementary air to be supplied in feedback control can be interrupted, resulting in an abrupt drop in the engine rpm and even in engine stall depending on the magnitude of the engine load.